1. Field of the Invention
The present invention relates to a polarization filter. Furthermore, the present invention relates to the application of a polarization filter to a light source, a sensor, a sensor system, and the like.
2. Description of the Related Art
A sensor system using polarized light is known in the related art. For example, JP2008-58270A discloses a technique of detecting cracks of a silicon substrate by using a system in which the silicon substrate is irradiated with polarized infrared light through a first linear polarization filter, and the reflected light or the transmitted light from the silicon substrate is received through a second linear polarization filter. The technique utilizes the following mechanism. That is, the reflected light or the transmitted light from a site free of cracks is linearly polarized light, and when such light passes through the second linear polarization filter, the amount of detectable light is reduced unless certain conditions are satisfied. In contrast, due to irregular reflection, the reflected light or the transmitted light from cracks generates detectable light even when passing through the second linear polarization filter. JP2003-96850A discloses an automatic water faucet device which detects a human hand or an object by using infrared light. In the device, erroneous detection is prevented by using first polarization means for transmitting a linear polarization component of emitted infrared light and second polarization means for transmitting a linear polarization component of received infrared light.
JP2013-368888A discloses a technique which uses circularly polarized light in the technique of JP2008-58270A. According to JP2013-368888A, because circularly polarized light is used, a polarization direction of a second linear polarization filter does not need to be adjusted.